The goal of this proposal is to understand the role of CD34 in hematopoiesis through molecular/biochemical means. Specific Aim 1 is to establish at what stage(s) of progenitor cell commitment and in which myeloid/erythroid lineages CD34 can regulate differentiation in vitro. Specific Aim 2 is to determine the significance of the intracellular regions of the full- length and truncated CD34 proteins. As a future study, the Applicant will evaluate the effect of over expression of CD34 on developmental hematopoiesis in vivo. The testing hypothesis is that inappropriate expression of CD34 may inhibit or negatively regulate the differentiation of hematopoietic progenitor cells.